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Research: Migration No Longer Best Strategy for Yellowstone Elk

June 5, 2013 — Migratory elk are coming back from Yellowstone
National Park with fewer calves due to drought and increased numbers of big
predators -- two landscape-level changes that are reducing the benefits of
migration with broader implications for conservation of migratory animals,
according to a new study published in the journal Ecology.

The new study by the Wyoming Cooperative Fish and
Wildlife Research Unit -- a joint program involving U.S.
Geological Survey (USGS), the University of Wyoming and the Wyoming Game and
Fish Department -- describes a long-term decline in the number of calves
produced annually by the Clarks Fork herd, a population of about 4,000 elk
whose migrants travel annually between winter ranges near Cody and summer
ranges within Yellowstone National Park.

Migratory elk experienced a 19 percent depression in
rates of pregnancy over the four years of the study and a 70 percent decline in
calf production over 21 years of monitoring by the Game and Fish Department,
while the elk that did not migrate, known as resident elk, in the same herd
experienced high pregnancy and calf production and are expanding their numbers
and range into private lands outside of the park.

“This is one of North America’s wildest and
best-protected landscapes, where elk and other ungulates still retain their
long-distance seasonal migrations -- and yet, it is the migratory elk that are
struggling while their resident counterparts thrive in the foothills,” says
Arthur Middleton, who led this work as a UW doctoral student and is now a
postdoctoral fellow at the Yale School of Forestry and Environmental Studies.

A key finding of the study was that only 70 percent
of migratory elk were pregnant, compared to 90 percent of residents -- a rate
more typical of Rocky Mountain elk. The study shows that the hotter and drier
summer conditions of the last two decades, coincident with the long-term drought
widely affecting the West, have reduced the duration of the spring period when
tender new grasses are available to elk. This makes it harder for female migratory
elk to find the forage they need to both nurse a calf and breed. Though elk
typically bear a calf every year, migratory elk that nursed a calf had only a
23 percent chance of becoming pregnant again in the following year.

Another likely cause of the declining calf numbers
among migrants was predation. Migrants share their range with four times as
many grizzly bears and wolves than resident elk, and both predators are well
known to prey on young elk calves. Resident elk get a break from high levels of
predation, in part, because when predators kill livestock on the resident range,
they are often lethally removed by wildlife managers and ranchers.

“A lower pregnancy rate reduces the number of calves
that are born in the first place. Predation seems to reduce the number of
migratory calves that survive the first few months of life,” says Matthew
Kauffman, a research wildlife biologist with the USGS and assistant professor
at UW. Kauffman explains that resident elk numbers are growing in the foothills
not because migrants are choosing to stay behind, but rather because irrigated
fields and lower predator numbers are allowing residents to raise more calves
to adulthood.

Globally, wildlife migration is a dwindling
phenomenon. Research and management often focus on conspicuous barriers like
fences, roads and other kinds of development that can physically impede
migration corridors. While those are important, this study suggests that even
in a landscape as well-protected as the greater Yellowstone ecosystem, subtler
changes in predator management and forage quality on the seasonal ranges of
migratory animals also will play an important role.

Migration is conventionally understood as a strategy
to gain better forage quality while also reducing exposure to denning
predators but, in this case, it seems those benefits are instead being realized
by the residents.

The study’s authors note that their work does not
predict that migratory elk will disappear, but rather that there could be a
long-term shift under way in the relative abundance of migratory versus
resident elk in the system. The study also highlights the perils of
characterizing Yellowstone wolf reintroduction as a “natural experiment.” Other
key factors have changed since wolves were reintroduced, including growth in
grizzly bear numbers and recurrent long-term drought associated with reduced
snowpack and hotter summers. The authors caution that such factors should be
taken into account in the effort to understand ongoing ecological changes in
Yellowstone.

Middleton also points out that this work highlights
the complex challenges facing regional wildlife managers and other stakeholders
as they continue to adapt to the reintroduction and recovery of large
carnivores, and severe drought that some studies suggest is linked to
longer-term climate change.

“Most immediately, these trends have meant lost
hunting opportunity in the backcountry areas frequented by migratory elk, and
increasing crop damage and forage competition with domestic livestock in the
front-country areas where resident elk are expanding,” he says.

This work was a collaboration among the USGS’s Wyoming
Cooperative Fish and Wildlife Research Unit, the Wyoming Game and Fish
Department, and the U.S. Fish and Wildlife Service, with cooperation from Yellowstone
National Park and other agencies. Primary funders include the Game and Fish
Department, the Rocky Mountain Elk Foundation, the Wyoming Animal Damage
Management Board, the Wyoming Governor’s Big Game License Coalition and USGS,
among others.

The study is featured in a Forum section of Ecology,
with a series of commentaries from other ecologists who study wildlife
migration and predator-prey interactions.